The invention relates to the field of safety shields and more particularly to an optically clear safety shield to be positioned over a vacuum or pressure equipment viewing port.
Vacuum and high pressure chambers are designed and constructed to handle large forces due to the pressure differential between the interior of the vacuum or pressure chamber and the exterior of the chamber (atmospheric pressure). It is common practice to include one or more viewing ports in the chamber to allow an observer to watch an experiment being performed within the chamber or to enable a photographic record of the experiment to be made. These viewing ports are normally formed integrally to and are airtight with the surface of the chamber, the window itself being formed from one or more thicknesses of an optically clear material, such as glass.
In a vacuum chamber, the interior of the chamber is held at a pressure considerably lower than atmospheric pressure. The forces acting on the walls and viewing ports of the chamber, therefore, will be directed inwardly toward the center of the chamber. In a high-pressure chamber, conversely, the interior of the chamber is held at a pressure higher than atmospheric pressure. Thus, the forces acting on the walls and viewing ports of the chamber will be outwardly directed.
In many vacuum and pressure chamber systems the chamber is cycled periodically to vary the vacuum or pressure applied to the interior of the chamber. After a number of vacuum or pressure cycles, the sealing arrangement of the viewing ports can become weakened or the window itself can become weakened due to the cyclic stresses. If the seal or window becomes sufficiently weakened the port will crack and shatter and, depending upon whether the air pressure is lower or higher than atmospheric within the chamber, implode or explode with considerable force. In either case, fragments of the shattered port will be propelled at high velocities presenting considerable danger to personnel in proximity to the chamber. In addition, any equipment disposed near the shattered viewing port will likely be damaged.
One prior art approach toward reducing the risk of a vacuum or pressure chamber viewing port or the like shattering under pressure is to fabricate a viewing port window of optically clear material which is supported by a metal grid to relieve stress on the window. In another prior art approach, an airtight viewing port is formed with two spaced-apart plates, one being flat and the other being curved. Other prior art approaches include providing a gas venting arrangement to prevent shattering of a dual window viewing port, and forming an impact resistant window assembly including a sheet of glass placed between a pair of spaced panes made of polycarbonate material.
While the described prior art vacuum and pressure chamber viewing ports do allow higher pressures to be sustained due to their "dual-window" or reinforced structure over those obtainable from single window viewing ports, the possibility still exists that such windows can shatter. In addition, these types of ports are costly to manufacture and can restrict the view through the port due to double refraction of light through the dual windows or interference by the reinforcing grid.
It is therefore an object of the invention to provide a safety shield for a vacuum or pressure chamber viewing port which affords a high degree of protection from flying fragments or debris resulting from accidental shattering of the port.
It is a further object to provide a safety shield for a vacuum or pressure chamber viewing port which does not restrict the view through the port.
It is another object to provide a safety shield which is easily manufactured and attached to a vacuum or pressure chamber viewing port.